8

Suppose you have a file that you wish to maintain two encrypted versions of, each version encrypted with its own (randomly generated, sufficiently long) key. The actual method of encryption is by creating two TrueCrypt (AES) volumes of the same size and copying the file into each of these volumes. The plaintext version of the file is then securely destroyed.

If an attacker gained access to both encrypted volumes, would he gain any advantage over a scenario in which he had access to only one of the volumes (assume he knows both volumes contain the same file)? i.e., is it safe to maintain two encrypted versions of the same file with different keys?

9

It is safe if the algorithm is not weak. If it was not safe then this would count as a serious break of the algorithm. No such weakness is known for (properly employed) AES. TrueCrypt has good reputation and uses a cryptographer-approved encryption mode.

So, yeah, no worry.

4

While it's true that it's not a problem for AES as Tom Leek correctly points out, some ciphers, most notably RC4, do have this problem. For RC4 at least, it is especially bad if the two keys differ by only a very small amount (for example, the nonce, which is sometimes concatenated with the key material for stream ciphers like RC4, differs). This is one thing that lead to WEP being cracked so completely.

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